Connecting large diameter tubulars, such as metal well bore tubulars which are lowered down hole in oil wells, has been problematic in the art. Large tubulars, for example tubulars comprising an external diameter of two feet or greater, are cumbersome and generally have to be handled by using powerful machines to grip and position the tubulars. Connecting such tubulars by means of threaded connections has been impractical because of the precision required in positioning and turning the cumbersome large tubulars. As a consequence of the difficulty and impracticality of using threaded connections for large tubulars, such tubulars are generally connected by way of welding. Even where an initial connection is made by threaded engagement, it is a common practice in the art to reinforce or secure the connection with weld beads.
To connect large tubulars by welding, the tubular members may be stationary, so no manipulation, turning, or precision placement of the tubulars is required. However, welding is very time consuming, expensive, and requires highly skilled personnel to perform. A connector for rapidly and securely connecting large tubulars by threaded engagement without the use of welding is useful to avoid the time, expense and expertise required for a welded connection.
Another problem is that the threads necessarily have a very large load and tend to be susceptible to forces, as discussed further hereinafter, that may distort the shape of the threads. Moreover, when initially starting the threaded contact the threads are easily damaged. Preferably, a threaded connection should be designed so that when made up the axial compression loads acting on the connection joint are not born solely by the threads of the connection; instead, the loads should be born as much as possible by the shoulders and nose faces of the connector so that the integrity of the threads is maintained. Further, a threaded connection should be able to be made up quickly.
Threaded connections between pipe members are typically made by providing one end of one pipe member with a male connector in the form of an externally threaded pin member, and providing one end of a second pipe member with a female connector in the form of an internally threaded box member which receives the pin member. The pin and box members may be integral parts of their respective pipe members, or may be added thereto by welding or threaded engagement.
In the past, several different types of threaded connections have been designed to manage the extreme compressive, tensile, and bending forces to which the connection is exposed. Several prior art designs incorporate internal and/or external mating shoulders and end faces on the pin and box members. As used in herein, the terms xe2x80x9cend facexe2x80x9d and xe2x80x9cnose facexe2x80x9d are interchangeable. In several designs, the mating shoulders are used as torque shoulders to stop axial advancement of the pin and box members during make up of the joint. In many designs, the mating shoulders are also used to provide resistance to axial compression during pile driving. Although many prior art designs use a combination of external and internal shoulders, these designs are usually configured such that only one of the shoulders will mate with its corresponding nose face upon initial makeup of the joint. These designs rely on either the external or the internal shoulder alone to mate with its corresponding nose face at initial make-up of the joint, with the other shoulder remaining axially spaced from its corresponding nose face at initial make-up of the joint. Some designs may never mate or have shoulders that only make contact with the corresponding nose face after the threads or other portions of the joint begin to yield.
It is therefore one object of the present invention to provide a threaded connection design that uses dual mating shoulders in which both external and internal shoulders mate with their corresponding nose faces during initial make-up of the joint. By providing dual mating shoulders, the shoulders share axial compression loads and provide the joint with improved performance in resisting the extreme axial compression loads encountered during pile driving.
In addition to providing resistance to axial compression loading, the dual mating shoulders in the present invention also function as torque shoulders to stop axial advancement of the pin and box members during make-up of the joint.
In several prior art designs, the threaded connections use converging or wedge-type thread flanks rather than shoulders to act as a torque stop. As used herein, the terms xe2x80x9cconvergingxe2x80x9d and xe2x80x9cwedge-typexe2x80x9d are interchangeable. In general, the pin and box threads in a converging thread flanks connection have progressively changing axial widths. The axial thread width of the pin member progressively decreases in the direction of the mouth of the pin member over the length of the thread structure. The axial thread width of the box member, on the other hand, progressively decreases in the opposite direction, such that a pair of pin and box members in the fully made up condition have a mutual wedging interfit. When converging threads are screwed together and wedging between the flanks takes place, the torsional resistance of the connection increases as the thread flanks act as torque stops to axial advancement of the pin and box members. Several other thread connection designs use tapered buttress-type thread forms that rely on radial interference to stop axial advancement of the pin and box members during make-up. In a tapered threads configuration, the radial interference fit forms as the crests and roots of the pin and box threads converge upon make-up of the joint.
Although these thread form designs may succeed in providing a torque stop to halt axial advancement of the pin and box members during make-up, and also allow the threads to provide resistance to axial compression loading, thus taking pressure off any pin and box shoulders that may be used in the design, such use of an interference fit in the thread form has its drawbacks. Such uses of interference fits in the form may create high surface contact stresses on the threads, which can cause galling and other localized thread damage that can severely limit the number of times the connection can be made up. In addition to limiting the repetitive use of the threads, the areas of high surface contact stress are susceptible to stress corrosion cracking, known as sulfide stress cracking, that occurs in petroleum well conduits. In one embodiment of the present invention, a threaded joint connection that uses the shoulders of the pin and box members rather than the threads functions as a torque stop.
Conventionally, the pin member of the joint is tapered inwardly from the proximal end of the threaded portion to the distal end to mate with a similarly tapered female threaded box member. The taper facilitates entry of the pin member in to the box member. Although the taper facilitates entry of the pin member, the wall thickness at the nose face end of a tapered thread form is often very small, especially in the flush joint configuration. Although the wall thickness at the shoulder of the pin and box member may be a substantial portion of the pipe wall thickness, with the shoulder occupying only a small portion of the wall, the wall thickness at the nose face end may be very small. This tapered configuration leaves the nose face end with a reduced wall thickness that must withstand the extreme axial compression during pile driving, as well as the extreme tensile, compressive, and bending forces. to which the pipe is exposed downhole.
Additionally, although a tapered thread form may facilitate entry of the pin and box members during make-up of the joint, tapered threads are still susceptible to cross-threading if the pin and box members are not properly aligned at the point of threaded engagement. One example of an apparatus designed to prevent cross-threading is found in U.S. Pat. No. 4,407,527, issued to Mr. Larry E. Reimert. The Reimert patent discloses a guide surface axially spaced from the internal threads of the box member to constrain the relative orientation between the pin and box members prior to threaded engagement. Although the Reimert design may be successful in preventing cross-threading, we have found that the guiding means may also integrate into a mating shoulder configuration by axially spacing the nose face from the thread on the pin and box members.
Those skilled in the art will appreciate the present invention that addresses the above and other problems.